The invention is in the field of furnaces; more particularly, the invention relates to a jet recirculation system for use in vacuum furnaces, such as vacuum carburizing furnaces.
The method and apparatus of the present invention were developed and intended primarily for vacuum carburizing furnaces. However, the present invention can be used wherever modification to atmosphere composition and/or circulation of the atmosphere within the furnace heating chamber are desired. The invention is particularly useful in furnaces operating under a vacuum or very low pressures. It can be used in batch or multi-zone furnaces.
Apparatus and methods of carburizing are well known in the art. Gas carburization takes place in a furnace under a vacuum. When used with reference to the carburization process, the term vacuum implies that the furnace chamber is evacuated and carburizing gas, such as natural gas or methane and/or a carrier gas, such as an endothermic carrier gas, are then fed into a chamber which is held at less than atmospheric pressure. The carburizing gas contains a certain amount of carbon or has a carbon potential. The carbon from the gas is absorbed into the steel.
It is well known that to provide uniform carburizing within the workload, especially a workload of higher packing density, circulation of the atmosphere within the furnace chamber is necessary. This can be accomplished with a fan or with jets. In vacuum furnaces fans are impractical because such furnaces are designed for temperatures as high as 2400.degree. F., where difficulties would be encountered in the material selection. Even at temperatures as low as 1700.degree. F., the reduced pressures require that fans provide greater velocities than normal in conventional furnaces. Jets for use to circulate the furnace atmosphere have no restriction other than that sufficient gas be provided to satisfy flow requirements.
During the carburizing process, the composition of the gas changes. The concentration of the carbon bearing gases starts to decrease as they react to release their carbon. Additional amounts of gas are then needed to replenish the furnace atmosphere. The amount of additional gas needed had to be arrived at emperically by trial and error. The amount of gas needed, i.e. carbon demand, varies depending upon the amount of work charged and the composition of the furnace atmosphere. It has been difficult, if not impossible, to monitor and control the composition of the atmosphere because of the reduced pressure.
In the past, to solve the problems of circulation necessary for uniform vacuum carburization and replenishment of the carburizing gas, a fresh supply of gas was continuously supplied. To achieve the necessary atmosphere circulation, gas jets had to be operated at high velocities, and, therefore, an excessive amount of carburizing gas was necessary for satisfactory operation of the carburizing process.
U.S. Pat. No. 3,796,615 reissued as RE. 29,881 by Westeren discloses a method of vacuum carburizing by replenishing the carburizing gas and providing additional recirculation. In the method described by Westeren, the pressure and, therefore, the concentration of carburizing gas in the furnace chamber is carefully controlled. Carburizing gas is supplied to the furnace chamber by a plurality of inlets at predetermined intervals. Westeren alternately introduces a carburizing atmosphere into the heating chamber and then evacuates it at predetermined cycles. "This so-called pulsing affect tends to remove unwanted molecules of the carburizing environment from around the part being treated; and upon reintroduction of the carburizing atmosphere into the heating chamber after evacuation thereof, the carbon and the carburizing gas will be more readily absorbed into the article. Further, the pulsing affect produces a better distribution of carbon around the article, and in certain articles that are formed with irregular surfaces, the pulsing technique is provided and the carburizing cycle ensures that sufficient carbon will be diffused into the metal to produce the required result" (Westeren, column 5, lines 42-53).
U.S. Pat. No. 3,128,323 by Davis discloses a system for the measurement and control of the constituent potential of gaseous atmosphere and has for an object the provision of a method and apparatus for determining the carbon potential of an atmosphere of a carburizing furnace in which a sample stream thereof has a carbon potential beyond the range of the filamentary ferrous metal detecting elements used in the art. In order to do this, Davis pumps a sample stream from the furnace, through a flow meter and through the carbon measuring apparatus of his invention. Based on the measured carbon potential, means are provided for the addition of enriching material, or decarburizing material for alteration of the carbon potential. The means for adding decarburizing material has particular use in the multi-zoned furnace used to illustrate the Davis invention. Further, the method and apparatus of the Davis patent are pertinent to positive pressure furnaces rather than the vacuum furnaces of the present invention.
The Westeren patent requires the amount of carburizing gas to be determined in advance. Westeren states at column 6, line 34, "prior to the beginning of the operation of the furnace and the carburizing process, the operator preselects the number of pulses that will be required to produce a selected carburized case and will also select the period of time for each pulse." Westeren continues at column 6, line 43, "The absolute pressure or vacuum at which the carburizing cycle is performed is also preselected . . . "
Methods of the prior art have two main drawbacks. The first is that they cannot provide a continuous circulation over the wide temperature range used in vacuum carburizing furnaces, particularly at temperatures over 1700.degree. F. without the introduction of excess gas. The second drawback is that in vacuum systems where gas is not continuously fed in for purposes of circulation, the amount of additional gas needed for carburization is arrived at emperically by trial and error and set in advance. This amount must vary depending upon the amount of work charged and the composition of the furnace atmosphere. Therefore, a need exists in the vacuum carburizing art for a method of continually monitoring the composition of the carburizing gas to determine the amount of gas used so as to be able to continually measure how far the carburization of the item treated has gone. Additionally, there is a need to provide circulation using a minimum of carburizing gas particularly when jets of carburizing gas are used as a circulation means.